I think they play exactly the same. The editor slipped up. The only difference in play is that you must *have* a sniper to play a Sniper as a discard. If you've lost your sniper or the scenario says you have no sniper then you can only discard your Sniper in the usual sense, you can't play it as a discard.

Maybe it came out different because the other cards are discarded onto the group while the Sniper actually does cause a particular procedure to be executed.

Yes, all these cards (Terrain, Wire and Sniper) are used as discards. At first sight there seems to be no reason for the difference in wording, but on thinking about it, it may be that the phrase "in lieu of a discard" was used because the Terrain and Wire cards go on the table, on the relevant group, and stay there until rejected or removed, whereas the Sniper card goes into the discard pile once its attack is resolved.

Perhaps the writer was trying to avoid the question "well if the Terrain is discarded how come it goes on a group? Surely it should go in the discard pile?"